Friday, June 13, 2014

Bias-Corrected Sea Ice Forecast

This is a follow-up to Monday's post about the latest CFSv2 model forecast of much higher Arctic sea ice extent this summer and autumn.  In that discussion I showed that the model is predicting that the September mean ice extent will be the highest since 2001, but this does not consider the bias in the model forecasts.  Numerical weather prediction models always contain a certain amount of systematic error or bias, and therefore any careful analysis of the forecasts requires a bias correction to be performed by comparing the latest forecast to the forecasts that have been made in the past.  To allow users to do this, NOAA helpfully provides a complete history of "re-forecasts" (emulated forecasts) back to 1982 for the CFSv2 model; so I downloaded the history of sea ice forecasts made on June 10 and obtained a complete history of forecasts for September mean Arctic ice extent.

The chart below shows the history of June 10 forecasts, along with the observed September ice extent according to the National Snow and Ice Data Center.  Ice extent was defined as the area covered by at least 15% concentration of sea ice in the model, which matches the NSIDC definition.  Interestingly the model forecasts have a low bias over the entire history - see the bottom of this post for a discussion of this.  However, in recent years the forecasts have failed to capture the extent of the melt-out, i.e. the June forecasts have predicted much less change over time than has actually happened.

If we compare the forecast and observed ice extent to the 1982-2010 mean of each series, then we have a bias-corrected comparison, see below.  The model failure in recent years really stands out, but it is also clear that there is some skill in predicting the year-to-year variability.  This year's forecast is also really dramatic, because the model is predicting the highest extent since 1992 when compared to itself.  It seems highly unlikely that anything of this magnitude will actually happen, but it also seems likely that the model is capturing some kind of signal.  Based on my experience in seasonal forecasting, it is usually worth paying attention when the models show large anomalies, although usually the timing, magnitude, or location of the predicted anomaly is not quite right.

In regard to the low bias in the model forecasts, at first glance this appears to be the opposite of the bias claimed by the model developers in their published article (as helpfully pointed out by Brian): "For the sea ice prediction, sea ice appears too thick and certainly too extensive in the spring and summer... The model shows a consistent high bias in its forecasts of September ice extent."  The figure below is taken from the article and shows (in the lower left panel) that the June 15 forecasts produce ice concentration that is too high compared to the observations.  However, ice concentration is not the same as ice extent, and it seems possible that the model could be producing ice that is too densely concentrated but yet the 15%-area is too small.  I haven't yet obtained a history of observed sea ice concentration to be able to test this idea.


  1. Thanks for following up on this Richard. The CPC has discussions that accompany their seasonal land-based forecasts but I don't see any similar product for the sea ice forecasts. Interestingly, the CPC's summer temperature forecast for all of Alaska is for above normal temperatures.

    In the long-lead prognostic discussion, the CPC states:


    I wonder if there is a correlation between ENSO onset and sea ice conditions.

  2. Great analysis Richard. I guess I thought the CFS output was bias correct from the hindcast climatology. Maybe not all fields. Hummm....

  3. A different ice/snow related question here:
    First some background: I live in Fairbanks but work in a remote mountain region in the northern southeast panhandle of Alaska. Saturday the mountains got fresh snow down to around 3500 feet overnight. Interesting for June but not unprecedented. I believe that the local ice fields in the area have a modifying effect on the local air masses especially in early summer. Tonight I am in Anchorage headed back to Fairbanks and I see fresh snow on the peaks around the area from the current little low that is tracking across the gulf. How normal is it for snow in June in the mountains around Anchorage? I used to climb these mountains when I was a teen 25 years ago and I'd say the snow level is down around 4000 feet. (The top of wolverine peak is frosted)
    What about snow in the white mountains say down to 4000 feet in June? (Although 4000 feet is a stretch for the hills of the interior as most are 2000 feet or less)
    Any takers?

    1. Mike, as an Anchorage resident, I can chime in on the Chugach Mountain snow. The snow on Wolverine peak actually fell a few days earlier. Areas above 3,700' received up to 6" of snow. On May 31st, up to 2' of snow fell above 2,500'. My experience has been that every few years there is a dusting of snow above 4,000' in early to mid June around Anchorage. Two years ago it happened on the day before the solstice. I have not seen any snow in July or August in those same locations.

    2. Mike, the best way to get at this for the interior might be to look at the freezing level during warm-season rain events. I'll put it on the to-do list.

    3. Now's a chance...big rain forecast for the Interior:


    4. Here's a news report regarding recent snow in the Alaska Range, per Richard's comments above about the freezing level during warm-season rain events:


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